1. Field of the Invention
The invention relates to an antenna unit that is used to communicate with a wireless tag (RFID: Radio Frequency Identification).
2. Description of the Related Art
Recently, a system that communicates with an article provided with a RFID to read and write information of the article is gathering attention. In such a system, an antenna is used to transmit and receive a signal to and from the RFID. It is preferable that impedance matching is established between the antenna and an electric supply line. However, in actuality, from various reasons, the impedance mismatch is caused. Owing to the mismatch, an amount of reflected waves increases to weaken radio waves irradiated into a space.
From a viewpoint of the antenna efficiency, the reflected waves are desirably as small as possible. Accordingly, the antenna is used with the return loss (reflection loss) caused by the reflected waves made small. However, the return loss varies depending on an operating frequency that is used to communicate with the RFID. That is, while the return loss is small at a certain frequency, when the operating frequency is changed to other frequency, the return loss becomes larger to deteriorate the efficiency.